No-crossover electron gun

ABSTRACT

This invention relates to an electron gun for use in storage, scan converter and vidicon type tubes which have a storage target. This electron gun is characterized by a beam of electrons having an energy distribution approaching that of a &#39;&#39;&#39;&#39;MaxwellBoltzman&#39;&#39;&#39;&#39; distribution characteristic of the cathode temperature. This energy distribution of the beam electrons is generally achieved by avoiding the crossing over of the beam through the use of a diode accelerating and control section with an apertured control grid operating at a positive potential with respect to the cathode.

United States Patent Corson July 8, 1975 NO-CROSSOVER ELECTRON GUN I Primary ExaminerRobert Sega] d R. f. [75] Inventor Rayar Carson Carlsbad Call Attorney, Agent, or FlrmW. H. MacAllister; R. H. [73] Assignee: Hughes Aircraft Company, Culver Hi City, Calif.

[22] Filed: July 9, 1973 [57] ABSTRACT [2]] Appl 77 This invention relates to an electron gun for use in storage, scan converter and vidicon type tubes which have a storage target. This electron gun is character- [52] U.S. C].2 313/449; 313/452 i by a beam of electrons having an energy distribw [51] Int. Cl. H01J 29/48; HOIJ 29 62 i approaching that f a MaxwelLBoltzman distri [58] held of Search 313/65 65 452 bution characteristic of the cathode temperature. This energy distribution of the beam electrons is generally [56] References cued achieved by avoiding the crossing over of the beam UNITED STATES PATENTS through the use of a diode accelerating and control 3,008,064 11/1961 Niklas et al. 313/82 R Section With an apertured Control grid Operating at 3 3,295,001 12/1966 Burdick et al. 313/82 R positive potential with respect to the cathode. 3,436,583 4/1969 Hughes 313/82 R 3,628,077 12 1971 Bossers 313/82 R 1 Clam, 2 Drawmg Flgul'es Grid 1 NO-CROSSOVER' ELECTRON .GUN

BACKGROUND. OF THE INVENTION The nature of the electric fields of an electron gun including a cathode at reference potential, a control grid at a substantial negative potential relative to said reference potential and an anode at a positive potential relative to said reference potential is strongly converging to the electrons being emitted by the cathode so that the stream of electrons focuses or crosses-over somewhere between the control grid and the anode. It has been shown that the energy distribution of the electrons in an electron beam which has been crossedover or which contains a cross-over is far from the ideal Maxwell-Boltzman distributioncharacteristic of the cathode temperature. Reasons advanced as to the cause of this phenomenon are that it is caused by plasma oscillations in the cross-over, electron-electron interactions or possibly other reasons. In any event, it has been shown that electron beams generated without a cross-over can be made to approach a Maxwell- Boltzman distribution characteristic of the cathode temperature. 7 r

In the operation of cathode ray type tubes having a storage target, the energy of electrons in the reading and writing beams is typically referenced to the respective sources thereof, i.e., the cathodes of the respective electron guns. Proper operation of these tubes requires that substantially all of the electrons in the respective beams have energy levels within a comparatively narrow range. More particularly, an electron beam may be used to discharge a storage surface to cathode potential (cathode stabilization) or, alternatively, its flow through an electrode may be controlled by a charge pattern on a dielectric storage surface which is close to cathode potential. If the energy distribution in the beam is substantially wider than the theoretical Maxwell-Boltzman distribution, the electrons with excessively high energy can land on the dielectric of the target and cause the target potential to go negative with respect to the desired value. In cases of this type, the wide energy distribution has a harmful effect on storage time and on the stability of cathode stabilization.

It is, therefore, an object of the present invention to provide an electron gun which will generate a beam of electrons with a narrow energy distribution to increase storage time and to improve the stability of cathode stabilization.

Another object of the present invention is to provide an electron gun capable of achieving higher resolution in cathode ray type tubes incorporating a storage target.

Still another object of the present invention is to provide a diode type electron gun for use in cathode ray type tubes having a storage target without an electrostatic field cross-over between the cathode surface and anode thereof.

A further object of thepresent invention is to provide an electron gun capable of generating a beam of electrons having an energy distribution approaching that of the cathode Maxwell-Boltzman distribution.

SUMMARY OF THE INVENTION In accordance with the present invention, a typical planar thermionic cathode is closely spaced to a planar anode wherein there is disposed a small hole or trimming aperture of the order of 0.002 inch in diameter.

Beyond this hole, only cylindrical accelerating elements disposed about the path of the beam are employed. In operation, electrons are drawn from the thermionic cathode through the small hole in the planar anode by means of a comparatively low positive potential relative to that of the cathode. The electric field at the aperture in the anode is such as to cause the electron stream entering the aperture to diverge slightly, and this diverging action is enhanced by space charge. The hole in the anode is tapered, getting larger on the side away from the cathode, to prevent interaction be tween the electrons in the beam and the wall of the hole. The lack ofa cross-over and the absence of interaction between the beam electrons and the wall of the aperture appears to be responsible for the substantial retention of the original Maxwell-Boltzman energy distribution characteristic, with a temperature close to the true temperature of the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a cross-section drawing of the nocrossover electron gun of the present invention; and

FIG. 2 shows a detail of the cathode-grid 2 aperture of the no-crossover electron gun of FIG. 1.

DESCRIPTION Referring now to FIG. 1 of the drawings, there is shown a cross-sectional view of one embodiment of the no-crossover electron gun of the present invention which comprises a cathode assembly 10 which includes a cylindrical sleeve 11 having a closed end 12 at the right extremity thereof, as viewed in the drawing, with electron emissive material 14, FIG. 2, deposited on the outer surface thereof to provide a planar cathode surface. Returning to FIG. 1, an electric heater 15, positioned within cylindrical sleeve 11, provides the heat necessary for the activation of electrons from emissive material 14 onthe planar cathode surface. Spaced concentrically about and substantially coextensive with the cathode assembly 10 is the cathode support cup which constitutes a cup-shaped electrode 16 with a circular aperture 17 in the center portion of the right extremity thereof, as viewed in the drawing, which has a diameter greater than that of sleeve 11 of the cathode assembly 10. Spaced from the cathode assembly 10 in the order named and in proper alignment along the path of electron travel are grid 1, grid 2, and grid 3.

Grid 1 constitutes a lens cup 18 including a cylindrical portion 19 of substantially the same diameter as that of the cathodesupport cup 16 with a circular aperture 20 in the center portion of the left extremity thereof, as viewed in the drawing, of a diameter substantially equal to that of sleeve 11 of cathode assembly 10. Lens cup 18 additionally includes a lip portion 21 at the extremity thereof farthest from cathode assembly 10 and a circular disc 22 of a diameter less than the diameter of aperture 17 of cup-shaped electrode 16 but larger than that of aperture 20. Disc 22 is disposed concentrically over aperture 20 in electrical contact with lens cup 18 and onv the side thereof nearest the cathode assembly 10. Referring to FIG. 2, disc 22 may, for example, have a thickness of the order of 0.010 inch and is spaced of the order of 0.006 inch from the planar cathode surface of cathode assembly 10. A hole 23 is disposed through the center of disc 22 and expands outwards from 0.002 inch on the side of disc 23 nearest the planar cathode surface to 0.010 inch on the oppo site side.

Referring again to FIG. 1, grid 2 constitutes a cylindrical electrode 28 stepped-down in the center region thereof from a diameter equal to that of lens cup 18 to a diameter small enough to allow electrode 28 to be inserted into lens cup 18 without contact. Lastly, grid 3 constitutes electrode 30 of substantially the same diameter as the larger diameter of cylindrical electrode 28 and extends from adjacent thereto to the right extremity of the electron gun as viewed in the drawing. The extremity of electrode 30 nearest electrode 28 is curved outwards and the opposite extremity is curved slightly inwards.

In operation, the following voltages are representative of those typically used:

Cathode potential volts Grid 1 potential 0 to +40 volts Grid 2 potential +400 volts Grid 3 potential +500 volts With the above potentials applied to the cathode sleeve 11 and the respective grids l, 2, and 3 a beam current of from 1 to 3 micro-amperes is produced beyond the aperture 23 in the disc 22 of the cup-shaped electrode 18. It is anticipated in this particular embodiment that the entire electron gun be immersed in a controllable axial magnetic field (not shown) which is adjusted to give one or more integral spirals of electrons with tangential velocity components between the aperture 23 and a target (not shown), thus focusing the beam on the target. It is evident that other focusing arrangements can be used without departing from the spirit and scope of the invention.

What is claimed is:

1. An electron gun consisting of a cathode assembly including a metallic sleeve having a closed end at one extremity thereof thereby to provide a planar surface, and electron emissivematerial on said planar surface, and a heating element within said sleeve for heating said electron emissive surface; a cup-shaped electrode disposed concentrically about and substantially coextensive with said sleeve with a centrally disposed aperture surrounding said planar surface means for providing a control electrode having an aperture therein disposed adjacent and parallel to said planar surface for producing an electron beam; and means for providing a plurality of cylindrical accelerating elements about the path of said electron beam.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,894,261

DATED July 8, 1975 INVENTOR(S) Bayard R. Corson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In item F57"Rayard R. Corson" should read:

--Bayard R Corson-a Signed and Sealed this thirtieth Day of Septemberl975 [SEAL] Artesr:

RUTH C. MASON C. MARSHALL DANN T 1 Commissioner pfPurenIs and Trademarks 

1. An electron gun consisting of a cathode assembly including a metallic sleeve having a closed end at one extremity thereof thereby to provide a planar surface, and electron emissive material on said planar surface, and a heating element within said sleeve for heating said electron emissive surface; a cupshaped electrode disposed concentrically about and substantially coextensive with said sleeve with a centrally disposed aperture surrounding said planar surface means for providing a control electrode having an aperture therein disposed adjacent and parallel to said planar surface for producing an electron beam; and means for providing a plurality of cylindrical accelerating elements about the path of said electron beam. 